By the 1700s, fossils had been inducted into the living world. Instead
of being produced by rocks themselves, fossils were recognized as the
remains of animals or plants. They looked too much like particular living
species
to be anything else. As the eighteenth century wore on, some fossils emerged
that could not be tied so neatly to the known living species. Elephants,
for example, had left fossils in Italy, where they could no longer be
found. Yet elephants still lived in Africa, and naturalists assumed that
other fossils had living counterparts of their own in some remote part
of the world. But, at the end of the century, a French naturalist popularized
an astonishing revelation: some species had actually vanished from the
face of the Earth.

Georges Cuvier (1769-1832) joined the fledgling National Museum in Paris
in 1795, and quickly became the world's leading expert on the anatomy
of animals. He then used that knowledge to interpret fossils with unprecedented
insight. Legend has it that sometimes even a few fragments of bones were
enough for him to reconstruct the complete anatomy of a previously unknown
species with uncanny accuracy.

This print shows the recovery of the first mosasaur fossils in 1780.
Cuvier used the fossils to support his radical ideas on extinction.

A few earlier naturalists, such as Buffon,
had argued that species might become extinct. But for some people
in Cuvier's day, the idea of extinction
was religiously troubling. If God had created all of nature according
to a divine plan at the beginning of the world, it would seem irrational
for Him to let some parts of that creation die off. If life consisted
of a Great Chain of Being, extending from ocean slime to humans to
angels, extinctions would remove some of its links.

A 1798 paper by Cuvier
contained this drawing showing the differences between the lower jaws of
a mammoth (top) and an Indian elephant. These differences supported the
idea that mammoths were indeed extinct.

Cuvier carefully studied elephant fossils found near Paris. He discovered
that their bones were indisputably distinct from those of living elephants
in Africa and India. They were distinct even from fossil elephants in
Siberia. Cuvier scoffed at the idea that living members of these fossil
species were lurking somewhere on Earth, unrecognizedthey were simply
too big. Instead, Cuvier declared that they were separate species that
had vanished. He later studied many other big mammal fossils and demonstrated
that they too did not belong to any species alive today. The fossil evidence
led him to propose that periodically the Earth went through sudden changes,
each of which could wipe out a number of species.

Cuvier established extinctions as a fact that any future scientific theory
of life had to explain. In Darwin's theory, species that did not
adapt to changing environments or withstand the competition of other species
faced annihilation. Darwin did not, however,
accept all of Cuvier's ideas on extinctions.
Like Charles
Lyell before him, he doubted that species
went extinct in great "catastrophes." Just as the planet's
geology changed gradually, so did its species become extinct gradually
as new species were formed.

Background extinction and catastrophe
On this score, Cuvier has been somewhat vindicated. Perhaps 99% of all
species that ever existed on Earth are now extinct. Most of those extinct
species disappeared in a Darwinian tricklewhat paleontologists call
"background extinctions." But several times over the past 600 million years,
life has experienced "mass extinctions", in which half or more of all species alive at the time
disappeared in fewer than two million yearsa blink of a geological
eye. The causes may include asteroids, volcanoes, or relatively fast changes
in sea level. These extinctions mark some of the great transitions in
life, when new groups of species got the opportunity to take over the
niches of old ones. Mammals, for example, only dominated the land after
giant dinosaurs vanished 65 million years ago in the Cretaceous-Tertiary
extinction. We humans, in other words, are the children of extinctions.

Lifes history has been marked by both catastrophic extinction events
(red spikes) and constant background extinction (yellow).